A wire wound electronic component has a structure as shown in FIG. 14(A). In this FIG. 14(A), at both ends of the core 10, whereon a wire is wound and whose shape is a column having a circle (oval, rectangular or similar shape) cross-section, has flanges 12 and 14 having a prism shape (or a plank shape with a rectangular cross-section) are formed. The core 10 and the flanges 12 and 14, all of which are made of magnetic materials, e.g., ferrite, form a coil bobbin 16. Electrodes 18 and 20 are formed on each side and end surfaces of the flanges 12 and 14 respectively.
The conductor 22 is wound on the core 10 formed at the central portion of the coil bobbin 16. The lead wires 24 and 26 at both ends of the conductor 22 are connected to the electrodes 18 and 20 respectively at each of side surfaces of the flanges 12 and 14. A concave portion formed between flanges 12 and 14 is coated with a sealing resin 28 to cover the conductor 22. The electrodes 18 and 20 whereto the lead wires 24 and 26 are respectively connected are applied with plates 30 and 32 respectively.
The above sealing resin 28 is made of, e.g., epoxy resin wherein ferrite powder is mixed as disclosed in Japanese Patent Laid-Open Publication No. 63-236305. Main components of the ferrite powder to be used for this purpose are, e.g., iron oxide, nickel oxide, zinc oxide, and copper oxide. Addition of the ferrite powder improves the magnetic shielding effect of the sealing resin 28 as shown by the dotted lines in FIG. 14, because it will allow a magnetic flux 34 to easily pass through inside the sealing resin 28. It will also enable to reduce magnetic effects exerted on the adjacent components, and improve the inductance of a wire wound electronic component.
When an electric current is passed through the conductor 22 of a wire wound electronic component having a structure described above, it generates heat. Ferrite used in the coil bobbin 16 and ferrite powder used as a filler for the sealing resin 28 changes the magnetic permeability (.mu.) thereof which shows the magnetic characteristics, along with its thermal changes. Accordingly, similar to other ordinary electronic components, it requires good thermal radiation. In terms of mounting, it requires a good countermeasure to prevent generation of static electricity for preventing the components from sticking to each other.
In addition, when there is not a close contact between the coil bobbin 16 and the sealing resin 28, as shown by the arrow FA in FIG. 14, water will enter into cracks of the component from the joint portion of the flange 12 (or 14) with the sealing resin 28, thereby reliability of the component will be deteriorated. The cracks will also cut the current of the magnetic flux 34 at the above joint portion, thereby the magnetic shielding effect and inductance will be lowered. If there is a pinhole 36 in the sealing resin 28, water will also enter inside the component as shown by the arrow FB.
If ferrite content is raised to increase the inductance, fine ferrite powder particles 38 may be strung from the surface of the sealing resin 28 to the coil bobbin 16 or the conductor 22. In this case, water might also enter into the component along with the surface of the strung fine ferrite powder particles 38 as shown by the arrow FC. The same thing might happen when the surface of the fine ferrite powder particles 38 do not have wettability towards the resin materials.
Ferrite powders filled into the sealing resin 28 are made by burning generally at high temperature over 1000.degree. C. Accordingly, the sintered ferrite substance becomes hard due to strong cohesion between the particles, and the size of the ferrite powder particles made by pulverization of the above ferrite substance might easily be varied. When those ferrite powder particles with uneven sizes are mixed into the above mentioned sealing resin for using an wire wound electronic component, it will suffer from several problems, i.e., deterioration of applicability of the sealing resin, unstable magnetic characteristic in the portion of the sealing resin, or similar problem. In other words, this would change the magnetic permeability in the portion of the sealing resin, thereby each component inductance would be changed. In addition, because a high internal stress will change inductance along with the change of magnetic permeability of the coil bobbin, as well as it would cause a damage of the coil bobbin, a break of the conductor or similar problem, relaxation of the internal stress might be desirable.
Filling ferrite powder into the resin will raise the viscosity of a sealing resin as a whole. When a sealing resin of high viscosity is used for coating and forming, the sealing resin will have less applicability than a sealing resin of low viscosity, and it will also requires high forming pressure. This high forming pressure accordingly exerts large stress on the coil bobbin 16 and conductor 22. This might cause a crack at, e.g., the joint portion of the flanges 12 and 14 with the core 10 because the portion is more vulnerable, or might cause a break of the conductor 22.
In addition, changes of stress will change the inductance. On one hand, this may cause deterioration of the component quality due to uneven quality, on the other hand, this may cause less manufacturing efficiency to produce the components with even quality.
The present invention, paying attention to the points stated above, has objectives to improve the thermal radiation, resistance against water and static electricity, and have better reliability of a wire wound electronic component.
Another objective is to obtain a sealing resin with stable magnetic characteristics and low internal stress by controlling the size of ferrite powder particles as a filler, as well as to retain good magnetic shielding effect.
In addition, another objective is to have efficient productivity of a wire wound electronic component with good quality by controlling change of inductance attributed to change of stress, as well as to protect a coil bobbin and wire by relieving stresses.